Conventional electronics devices including FA (Factory Automation) apparatuses such as a copying machine or a printer, OA (Office Automation) apparatuses, and amusement apparatuses are often required to detect various objects (recording papers, coins, balls etc.) transiting through a path inside the electronic devices. A photointerrupter that is a light detecting device, a device using light for detecting an object, is preferably used for such detection because of its capability of detecting substances without physical contact.
FIG. 3 illustrates a conventional photointerrupter (first conventional photointerrupter), which includes a light emitter 32 for emitting light, and a photoreceptor 33a for detecting the light. With this structure, presence or absence of substance is determined based upon whether the light across the light emitter 32 and the photoreceptor 33a is blocked. Usually, a light emitting diode that emits infrared light or visible light is adopted as the light emitter 32.
To detect light from the light emitter 32, the photointerrupter of this example includes: the photoreceptor 33a that is a photodiode for receiving light from the light emitter 32 and converting the light into current; an I-V converting circuit 33b that converts the current into voltage; a hysteresis-type comparator circuit 35 that compares the voltage with a reference voltage; and an output circuit 36 that externally outputs the voltage in appropriate condition. The reference voltage is generated by a constant voltage circuit 38 driven by a source voltage Vcc.
There are two types of photointerrupters: a transmission type photointerrupter and a reflection type photointerrupter. In the transmission type photointerrupter, when no object exists in an optical path across the light emitter and the photoreceptor, light from the light emitter is sensed by the photoreceptor. On the other hand, when an object exists in the optical path, the light from the light emitter is blocked. In the reflection type photointerrupter, it is the other way round.
As illustrated in FIG. 3, in the conventional photointerrupter, when the light emitter 32 emits light, that is, when the source voltage Vcc is supplied, a current IF is kept running through the light emitting diode. Then, it is determined whether an object transits the optical path across the light emitter and the photoreceptor.
Normally, the current IF in the light emitting diode functioning as the light emitter is approximately 10 mA to 20 mA. A current in the photodiode functioning as the photoreceptor is approximately one tenth of the light emitting diode. The power loss P of the entire photointerrupter is expressed as the formula (1) below:P=((Vcc−VF)/R1)×Vcc+Vcc×Icc   (1).In the formula (1), R1 represents a resistance of a current-limiting resistor for the light emitting diode, Vcc represents a source voltage, VF represents a forward voltage of the light emitting diode and Icc represents a current consumed by the photoreceptor (including an oscillation drive circuit).
For example, assumed that Vcc=5V, IF=20 mA, and ICC=2 mA, the power loss P1 in the light emitter is calculated as P1=5V×20 mA=100 mW, and the power loss P2 in the photoreceptor is calculated as P2=5V×2 mA=10 mW. It is apparent therefrom that the power loss in the light emitter is considerably higher than that of the photoreceptor.
FIG. 4 illustrates a circuit of a light-modulation type photointerrupter (second conventional photointerrupter). This photointerrupter detects an object in the way described below. In the light modulation type photointerrupter, the light emitter 32 emits a light pulse that corresponds to a pulse signal from an oscillation drive circuit 39, so as to prevent an error operation caused by disturbance light, such as DC light. Then, the photoreceptor 33a senses a light pulse and converts the light pulse into current, the current is converted into voltage, and the voltage is filtered by a BPF (band pass-filter) 40. Consequently, only a signal that passed through the BPF 40 is integrated by a waveform shaping circuit 41 and then is processed by the hysteresis-type comparator circuit 35. (see, for example, Japanese Unexamined Patent Application No. 225282/1995 (Tokukaihei 7-225282, published on Aug. 22, 1995)).
The circuit structure disclosed in the above publication (JP 225282/1995) can reduce a loss in the light emitter. However, because the circuit structure uses plural different logics, the size of the circuit is expanded, resulting in an increase of cost.